1. Field of the Invention
My invention relates to electric lamps and pertains more specifically to those well suited for automotive headlamp and like lighting applications. Still more specifically, my invention concerns an electric lamp unit comprising a single-filament bulb in conjunction with facilities for selective production of upper and lower beams of optimal cross-sectional patterns and intensity distributions. By the "upper beam" I mean, of course, a beam intended primarily for distant illumination when the vehicle is not meeting or following other vehicles, and by the "lower beam" a beam for illuminating the road ahead of the vehicle when it is meeting or following another vehicle.
2. The Prior Art
Automtoive headlamp systems are classifiable into two categories according to the number of lamp units, that is, those employing two units and those employing four units. Some four-lamp systems incorporate lamp units that are devoted exclusively for the production of the lower beam. A familiar example of such lower beam lamp units employ a lower beam shade positioned intermediate an ellipsoidal reflector and a converging lens. The ellipsoidal reflector has a first focus at which a bulb is disposed, and a second focus spaced farther away from the reflector than is the first focus. Disposed close to the second focus of the reflector, the shade functions to cut off the rays that have been reflected from the lower half of the reflector and which, consequently, are angled upwardly. Only the rays that have been reflected from the upper half of the reflector are allowed to bypass the shade and to impinge on the converging lens, thereby to be projected as the lower beam.
My evaluation of this known lower beam lamp unit as such is very favorable by reason of the good beam pattern obtainable with the provision of the shade only, which is simple in construction and easy of manufacture and mounting. As an additional advantage, the contoured edge of the shade provides a clearcut upper edge of the lower beam pattern and so effectively protects the drivers of the oncoming vehicles from glare.
Offsetting all these advantages, however, is the fact the lower beam lamp unit as so far constructed has lent itself to use only as such, namely, only as a unit of a four-unit headlamp system. The utility of this type of lamp unit will certainly be enhanced if it is switchable to provide both upper and lower beams, so that the lamp unit may find use in two-lamp systems as well.
I have contemplated the adaptation of the lower beam lamp unit for both upper and lower beam production by making the shade movable vertically relative to the reflector and the converging lens. The shade might be so moved between an upper working position, in which it cuts off the rays reflected from the lower half of the reflector, and a lower retracted position where the shade permits all the reflected rays to travel therepast. Not only the pattern of the lower beam produced when the shade is in the working position, but also that of the upper beam emitted when the shade is in the retraction position, would be of acceptable outline.
A problem arises, however, because of the unvaried vertical position of the "hot zone" (i.e. region of maximum light intensity) of the upper and lower beam patterns so produced. Generally, the hot zone of the upper beam must be located on the horizontal axis passing the center of the lamp unit, whereas the hot zone of the lower beam must be below the horizontal axis. If the above suggested adaptation of the known lower beam lamp unit is so optically configured that the hot zone of the lower beam is below the horizontal axis, the hot zone of the upper beam will be too low, being at the same height as that of the lower beam. Conversely, if the optical configuration is such that the hot zone of the upper beam is on the horizontal axis, then the hot zone of the lower beam will be too high.